Jelle and Dietsje Jolles published a review in Frontiers in Psychology on the question mentioned in this title.  They see 3 reasons:

1… “research in the past fifteen years has placed the emphasis on the results of experiments in which brain imaging methods (notably Magnetic Resonance Imaging, MRI) have been used. In retrospect, the neuroimaging experiments have yielded interesting scientific results, which have deepened our understanding of brain mechanisms underlying cognitive and affective processes. Yet, the fundamental and unidimensional nature of most imaging studies prevents a direct application to the field of education.”

2. …”there is a major lack of valid sources of information (books, articles, courses, internet sources etc.) for use by the interested educator.”

3. …”In the third place, there is a substantial confusion of tongues with respect to the potential importance of neuroscience and cognitive science knowledge and insights. This is evident in opinions that are expressed in statements such as “a bridge too far,” “beware of the brain hype” and visions stating that teachers do not need anything more than a good behavioral observation.”

This leads the authors to recommendations, such as:

Future research should take a transdisciplinary approach to take on problems and questions from the field of education, investigating the same issue on multiple levels of analysis. Thereby, neuroimaging research, laboratory studies with well-controlled behavioral tasks, and classroom studies could mutually inform and constrain one another. Still, at present, there is relevant knowledge about the learning brain, which appears to lie in an improved understanding of how to bring the brain in an optimal condition to learn, and by stimulating insight into external, non-psychological factors which act upon the learning individual. The vast amount of knowledge about “brain plasticity” and related topics yields predictions that could help to optimize the conditions for information processing and learning. Educational interventions in which sleep and fatigue, nutritional status, attentional processes or movement are manipulated are examples of approaches that may prove of value and deserve the attention of educational professionals. Nonetheless, we would like to re-emphasize that neuroscientific insights need to be combined with insights from other domains to form hypotheses about learning in the daily context. Educational researchers may play an important role in testing these hypotheses, enabling the conversion of true scientific insights into scalable practical applications.

Abstract of the review:

New findings from the neurosciences receive much interest for use in the applied field of education. For the past 15 years, neuroeducation and the application of neuroscience knowledge were seen to have promise, but there is presently some lack of progress. The present paper states that this is due to several factors. Neuromyths are still prevalent, and there is a confusion of tongues between the many neurodisciplines and the domains of behavioral and educational sciences. Second, a focus upon cognitive neuroimaging research has yielded findings that are scientifically relevant, but cannot be used for direct application in the classroom. A third factor pertains to the emphasis which has been on didactics and teaching, whereas the promise of neuroeducation for the teacher may lie more on pedagogical inspiration and support. This article states that the most important knowledge and insights have to do with the notion of brain plasticity; the vision that development is driven by an interaction between a person’s biology and the social system. This helps individuals to select and process information, and to adapt to the personal environment. The paper describes how brain maturation and neuropsychological development extend through the important period of adolescence and emergent adulthood. Over this long period, there is a major development of the Executive Functions (EFs) that are essential for both cognitive learning, social behavior and emotional processing and, eventually, personal growth. The paper describes the basic neuroscience knowledge and insights – or “neuroscientific literacy” – that the educational professional should have to understand and appreciate the above-described themes. The authors formulate a proposal for four themes of neuroscience content “that every teacher should know.” These four themes are based on the Neuroscience Core Concepts formulated by the Society for Neuroscience. The authors emphasize that integrating neuroscientific knowledge and insights in the field of education should not be a one-way street; attempts directed at improving neuroscientific literacy are a transdisciplinary undertaking. Teacher trainers, experts from the neuroscience fields but also behavioral scientists from applied fields (notable applied neuropsychologists) should all contribute to for the educational innovations needed.